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<HTML><!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN"><!-- Created on September, 12 2004 by texi2html 1.64 --><!-- Written by: Lionel Cons <Lionel.Cons@cern.ch> (original author) Karl Berry <karl@freefriends.org> Olaf Bachmann <obachman@mathematik.uni-kl.de> and many others.Maintained by: Olaf Bachmann <obachman@mathematik.uni-kl.de>Send bugs and suggestions to <texi2html@mathematik.uni-kl.de> --><HEAD><TITLE>Autoconf, Automake, and Libtool: Discarding input</TITLE><META NAME="description" CONTENT="Autoconf, Automake, and Libtool: Discarding input"><META NAME="keywords" CONTENT="Autoconf, Automake, and Libtool: Discarding input"><META NAME="resource-type" CONTENT="document"><META NAME="distribution" CONTENT="global"><META NAME="Generator" CONTENT="texi2html 1.64"><script language="Javascript"><!-- // Check the browser version. function checkVersion() { if (navigator.appVersion.charAt(0)>=3) return true; if (navigator.appVersion.charAt(0)>=4) return true; else return false; 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The <CODE>dnl</CODE>macro takes no arguments and expands to the empty string, but it has theside effect of discarding all input up to and including the next newlinecharacter. Here is an example of <CODE>dnl</CODE> from the Autoconf sourcecode:</P><P><TABLE width=100%><tr><td> </td><td class=example bgcolor=#6688aa><br><pre># AC_LANG_POP# -----------# Restore the previous language.define([AC_LANG_POP],[popdef([_AC_LANG])dnlifelse(_AC_LANG, [_AC_LANG], [AC_FATAL([too many $0])])dnlAC_LANG(_AC_LANG)])</pre></td></tr></table></P><P>It is important to remember <CODE>dnl</CODE>'s behavior: it discards thenewline character, which can have unexpected effects on generated<TT>`configure'</TT> scripts! If you want a newline to appear in theoutput, you must add an extra blank line to compensate.</P><P><CODE>dnl</CODE> need not appear in the first column of a given line -- itwill begin discarding input at any point that it is invoked in the inputfile. However, be aware of the newline eating problem again! In the exampleof <CODE>AC_TRY_LINK_FUNC</CODE> above, note the deliberate use of <CODE>dnl</CODE>to remove surplus newline characters.</P><P>In general, <CODE>dnl</CODE> makes sense for macro invocations that appear ona single line, where you would expect the whole line to simply vanishfrom the output. In the following subsections, <CODE>dnl</CODE> will be usedto illustrate where it makes sense to use it.</P><P><A NAME="Macro management"></A></TR></TABLE><BR> <FONT SIZE="-1">This document was generatedby <I>Gary V. Vaughan</I> on <I>September, 12 2004</I>using <A HREF="http://www.mathematik.uni-kl.de/~obachman/Texi2html"><I>texi2html</I></A></BODY></HTML>⌨️ 快捷键说明
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